open center theoretical question

[thebonepile]

I have a theoretical and maybe a terminology question....
I know the difference between open and closed center hydraulics.....

This is my question....

When I start my open center system up and it is just idling (power steering not in this discussion) and there is no demand on any hydraulic item, what device, if any, is regulating the pressure of the open stream, is it just going round and round from sump to filters to pump through control valves and back to sump at essentially zero pressure? i realize once i pull a control valve it sets up a different path for the oil to flow (use a hydraulic cylinder as an example) and once that cylinder is fully extended if I continue to hold the lever then the pump keeps buildng up pressure until the emergency relief pressure is achieved and the emergency relief valve opens to relieve that pressure....so is there a device which is used to set "no use" system pressure? what would it be called?

 

[AKKAMAAN]

Hydraulic power is the product of flow and pressure....zero or low pressure >>>zero or low power.....flow is "constant"!

When I start my open center system up and it is just idling (power steering not in this discussion) and there is no demand on any hydraulic item, what device, if any, is regulating the pressure of the open stream, is it just going round and round from sump to filters to pump through control valves and back to sump at essentially zero pressure?

Do you really want any pressure in an open center system when there is no demand of "action"???

i realize once i pull a control valve it sets up a different path for the oil to flow (use a hydraulic cylinder as an example) and once that cylinder is fully extended if I continue to hold the lever then the pump keeps buildng up pressure until the emergency relief pressure is achieved and the emergency relief valve opens to relieve that pressure....so is there a device which is used to set "no use" system pressure?

I would say that the pressure relief valve, PRV, will not relief the pressure, it will by pass the flow at the actual pressure setting. Pressure will stay the same as long as the is open....
If you have another control valve, connected to a cylinder with a lower pressure load, connected to this system, and you pull that lever, the pressure will drop, and the PRV closes, the flow will go to this next cylinder till it stalls as well.

There is no device for "no use" in the system....if the control valve is in neutral position (open center), thats the only "no use" device system will need.....

Your post shows you have it all figured out now!!:thumbsup:

BTW, please explain for us whats the difference between open and closed center systems!! It is a good basic thread!!

 

[thebonepile]

OK, this is the next step to the question

I have a White 2-75 (Iseki 9000) open center - hydraulics are all good - except the PTO engages hydraulicly, and it doesn't unless i hit a control lever and block a little flow, then it takes off. I have the service manual, and the system is supposed to maintain a secondary circuit pressure with the use of a spring and plunger and indexed holes, and such......(this secondary pressure is about 210 (I would have to check - maybe 185)) BUT if the pressure in the whole system is at 0, then it won't be able to push the spring and build up to the 185 - so back to my main question - if there is no hydraulic demand is it possible my open center system is always running at a certain pressure (like greater than 200)?

 

[kennyd]

is it just going round and round from sump to filters to pump through control valves and back to sump at essentially zero pressure?

You won't see "0" pressure, but probably around 500 PSI (WAG on my part) or so due to parasitic loss in the lines, hoses, fittings, and valves.

 

[AKKAMAAN]

OK, this is the next step to the question

I have a White 2-75 (Iseki 9000) open center - hydraulics are all good - except the PTO engages hydraulicly, and it doesn't unless i hit a control lever and block a little flow, then it takes off. I have the service manual, and the system is supposed to maintain a secondary circuit pressure with the use of a spring and plunger and indexed holes, and such......(this secondary pressure is about 210 (I would have to check - maybe 185)) BUT if the pressure in the whole system is at 0, then it won't be able to push the spring and build up to the 185 - so back to my main question - if there is no hydraulic demand is it possible my open center system is always running at a certain pressure (like greater than 200)?

I hope you are referring to pressure in PSI, and not BAR....and you need to post a symbol schematic drawing over the system....

Like kennyd posted, there is always some static pressure if there is flow....how big that pressure is depends on a few factors, like restrictions, flow and viscosity. At flow, pressure will show the highest number by the pump outlet, and pressure will drop proportionally to what restrictors there are....lowest pressure at inlet to reservoir....
so at low rpm idling, you might have as low as 100-200psi, while pressure can reach 500psi at full rpm's (max flow)

 

[J_J]

The pressure required for the hyd PTO, could be part of the lever/valve setup, by allowing fluid to flow through a restrictor, once the lever is activated. The pressure builds up and activates the hyd PTO. Release the lever, and pressure goes to zero, and PTO stops turning.

Pressure through the system should remain at a low figure until another valve is activated.